Conventionally, in an engine for a vehicle mainly traveling with wheels, such as a car, a truck, a wheeled loader, a wheel crane, a supercharger has been used to reduce the engine size while increasing the output of the engine, securing good acceleration. As for the supercharger, there is a mechanical supercharging method which uses part of the engine output, or the power of other machinery, and there is a turbocharger supercharging method which uses exhaust gas. For this reason, as the first background with the mechanical supercharging method, a supercharger 40 is mechanically and directly connected to and driven by an engine 41 through gears 42 and 43, or through a belt or the like, as FIG. 14 illustrates. In the directly connected and driving type where air is supplied as illustrated in FIG. 15, in an area A where the engine speed is lower with a higher load, the amount of air which is supplied to the engine is insufficient. In an area B where the engine speed is higher with a lower load, the amount of the air supply is excessive, increasing the driving loss of the supercharger 40.
There has been a differential driving method proposed for driving the supercharger 40 as shown in FIG. 16. In the differential driving method (I) in FIG. 16, a planetary carrier 43 is fixed to and connected to an output shaft 41a of the engine 41, and usually, three of planetary gears 44 are rotatably attached to the planetary carrier 43 equidistantly from one another. A sun gear 45 is meshed with the inner side of the three planetary gears 44, and a ring gear 46 is meshed with the outer side of the three planetary gears 44. At the inside of the sun gear 45, the output shaft 41a of the engine 41 is rotatably disposed. A gear 47 is provided at one end of the sun gear 45, and a driving gear 48 for the mechanical supercharger 40 is meshed with the gear 47. A shaft of a power transmission system 50 for a vehicle is connected to the ring gear 46, and the mechanical supercharger 40 is variably rotated, depending on the load of the power transmission system.
A differential driving method (II) in FIG. 17, for driving a supercharger by a differential driving method similar to the above, has a configuration substantially similar to the above. In the differential driving method (II), the ring gear 46 is provided with a brake 46a which utilizes electricity, dry condition, oil pressure, or the like. The rotational force of the mechanical supercharger 40 is controlled based on the difference in strength of the drag resistance caused by the brake 46a.
Next, as a second background, there is construction machinery which requires higher output at lower and medium engine speed, or there is construction machinery which requires higher output at higher engine speed, depending on the kind of machine used. For example, there is machinery, such as hydraulic shovel or the like, which needs higher output in an area Qa with the engine rotating at higher speed, as shown in FIG. 18. There is machinery, such as a bulldozer, a damp truck, a wheeled loader, a motor grader, or the like, which requires higher output at lower and moderate engine speed as shown in FIG. 19.
When classifying the uses of each machine's engine according to each load, the engine speed, and the frequencies of use, the construction machinery's engine described above is more frequently used in areas Fa and Fb of FIG. 20. In the vehicles with tires, such as a car, a truck for transporting cargo, a bus, a dump truck, a forklift, and the like, and in a fishing boat, a pleasure boat, and in a ship, the engines are more frequently used in areas Fb and Fe. In a word, in construction machinery, the engine is more frequently used under a medium or a higher load, while in a vehicle with tires and in a ship, the engine is more frequently used under a lower or a medium load.
Further, as a third background, in machinery which requires a higher engine output in the entire area, there is a method using both a turbocharger and a mechanical supercharger. In this case, as FIG. 21 shows, at the lower and medium engine speed, a mechanical supercharging method is used to produce higher output as shown by a solid line Ua, while at higher engine speed, the mechanical supercharger is stopped to reduce driving loss, and a turbocharger is used to produce a higher output as shown by line Ub of alternating long and short dashes.
A dotted line Uc shows the output torque of the engine in a non-supercharging condition. Lines Ya and Yb of alternating long and two short dashes, show matching lines of a torque converter when driving a vehicle using a torque converter. The alternating long and two short dashes line Ya shows e=0, while the alternating long and two short dashes line Yb shows e.apprxeq.1. Incidentally, in the drawing, the axis of abscissa shows the engine speed, and the axis of ordinate shows the output torque (load) of the engine.
The disadvantages in the first background which are desirably eliminated are as follows:
(1) The directly connected and driving type of supercharger in FIG. 14 is more frequently used for an engine with a lower or medium load in a vehicle with traveling tires, and in a ship. In an air supplying condition which is required for an engine with a lower or medium load, this type frequently causes an excessive and unnecessary driving loss of the supercharger. As a result, fuel consumption per hour is greater as compared to a non-supercharged engine or a turbosupercharged engine.
(2) In the conventional differential driving method (I), a load on a vehicle is great while the vehicle is being accelerated from a standing start (acceleration from a standstill); therefore, the rotational speed of the mechanical supercharger increases at first, and after a sufficient amount of air enters the engine, the vehicle starts to travel. Accordingly, an operator receives a slow response in the early stage of pressing down on the accelerator; specifically, an operator has complaint of poor responsiveness.
(3) In the differential driving method (II), part of the engine driving force is lost as a thermal loss due to the dragging resistance of the brake and the like, resulting in a disadvantage that the fuel efficiency is decreased.
The disadvantages in the second and the third backgrounds are as follows:
(4) As FIG. 21 shows, in the method using both a turbocharger and a mechanical supercharger, in a truck for transporting cargo, a bus, a dump truck, a folk lift, or the like, which uses a torque converter, the torque converter (e=0) does not match the higher output Ua produced by a mechanical supercharging method, and there is a disadvantage that the output of the supercharging method cannot be used. In addition, at a higher speed of the engine, a higher output is produced by a turbocharger as shown by the alternating long and short dash line Ub, but an engine in a much smaller size with much higher output and efficiency is desired.